Top hat stringer without core foam

How does core foam under hot hat stringer work? I know we usually use core materials for stringer and laminate mat or woven roving on it. What if don't use core form under the laminates. I saw a molded stringer without any foam in it.
Doesn't it affect to shear area? Section modulus of the stringer is depend on shape of the top hat which is upper side of the stringer. Do you think the molded stringer without any core foam is better than with core materials?

 

I've seen those done in large Cat's & Tri's. (performance boats). They usually reinforce the part with carbon fiber inlays. A structural engineer would know what affect it'd have leaving out the foam core but I assume they've (designers/engineers) have tested the relative strength of the two and decided that the added structural benefit of the foam isn't worth the weight. I doubt very much it's an economical decision to leave it out.

 

Usually the foam is used as a form for the laminate, and is just left in place--most likely the contrubution of the foam is neglected. Shear is carried by the web portion of the hat section.

 

I have been thinking of this subject too. We have been making some hulls for 5m RIBs which have been reinforced with traditional bulkheads and stringer glassed over. I was considering molding a grid and bonding it to the hull before installing the deck. I have seen this kind of grid used on larger sailboats, but never seen one in a power boat and have wondered why not? Some colleagues have suggested the time and expense to create the mold would not be worthwhile. My thoughts are that it has to be quicker to mold a part like this rather than cut and fit the cores, fit them in the hull and glass over. I haven't gotten around to any design or calculations yet, but it would be interesting to hear from someone who may have already considered this idea.

db